How can I make a class recognize that an interface found in a separate file exists

I've made an interface called ApprovalEvent in a separate file with 开发者_如何学JAVAthe namespace myproject... I've also made a class called PurchaseOrder... it's also in the same namespace, just in a separate file. Whenever I try to make PurchaseOrder implement ApprovalEvent it always says that ApprovalEvent is undefined...

How can I make the class recognize that the interface exists but is in a different file?

Here's some code:

File #1

Namespace myproject
    Public Interface ApprovalEvent
        Function getDetails() As String
    End Interface
End Namespace

File #2

Namespace myproject
    Partial Class PurchaseOrder 
        Inherits Entities.Modules.PortalModuleBase 
        Implements ApprovalEvent

        Function getDetails() As String Implements ApprovalEvent.getDetails
            return "details"
        End FUnction
    End Class  
End Namespace

Thanks,

Matt

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Separate project too? Does the one project include the other as a reference if so? That shouldn't be a problem from what I can see. It works no problem. Try temporarily including the interface in the same file too, see if that helps.